Role of Pr-Vacancies and O-Interstitials on the Activity and Stability of (Pr1-xLnx)2NiO4 (Ln = La, Nd, Pm, Sm, Gd, Tb, Dy, and Ho) towards Oxygen Reduction Reactions: A DFT Study

Praseodymium nickelate, Pr2NiO4 (PNO), is a promising electrode to promote oxygen reduction reaction (ORR) in a solid oxide fuel cell, but it exhibits phase transformation during electrochemical operation. The origins of the simultaneous phase transformation and high electrochemical performance still remain obscure. We carried out a systematic density functional theory study to elucidate the mechanism for this conjugated phenomenon. Both electronic structure/charge and normal-mode analysis suggest the presence of peroxide. Our study shows that the formation of peroxide (O22-) is attributed to both oxygen interstitials and Pr vacancies. The peroxide species limits the oxygen ion migration due to the additional energy required to break its O-O bond, which leads to a decrease in ORR activity. Subsequently, we investigate the diffusion paths of Pr-ions while comparing them with those of other Ln3+ ions (La, Nd, Pm, Sm, Gd, Tb, Dy, and Ho) in PNO. The formation energies for various Ln3+ cation occupancies are calculated, as well as segregation energies in CeO2(111) surfaces. Finally, criteria for effective Ln3+ dopants are developed. La, Nd, and Pm are proposed as potential substituents in PNO to obtain a stable structure.

Doping Effect of Alkaline Earth Metal on Oxygen Reduction Reaction in Praseodymium Nickelate With Layered Perovskite Structure

Pr1.9A0.1NiO4 (A = Ca, Sr, Ba) are synthesized and characterized by X-ray powder diffraction (XRD), infrared spectrum (IR), and X-ray photoelectron spectroscopy (XPS). The effects of alkaline earth doping on the covalence of Pr–O and Ni–O bond, the mean valence of Ni, and the hydroxide absorption ability of material surface are studied. It is found that the covalence of Pr–O and Ni–O bond increases with the decrease of alkaline earth element radius. Meanwhile, the mean valence of Ni and the surface hydroxide absorption ability are enhanced. The electrochemical measurement results indicate that the O2−2 /OH− replacement reaction is facilitated by the increase of mean valence of Ni in the material. The best oxygen reduction reaction (ORR) activity is found in Pr1.9Ca0.1NiO4. The current density of 2.16 mA cm−2 is obtained at a potential of −0.6 V (versus Hg/HgO). The tafel slope is 66.48 mV decade−1, close to Pt/C material.